What are the causes of bubbles on the cross section of foamed plastic sheet

The production of foamed plastic sheet generally adopts three different PVC foaming regulators, namely, heating type, heat absorbing type or heat absorbing and exothermic compound balanced type. The decomposition temperature of PVC foaming regulator is high, reaching 232 ℃, which is far higher than the processing temperature of PVC. The decomposition temperature should be reduced when using it. Therefore, PVC foaming regulator is generally selected for foaming adjustment of PVC materials. This kind of foaming regulator has high foaming rate, about 190-260ml/g, fast decomposition speed, great heat release, but short foaming time and strong suddenness. Therefore, when the amount of PVC foaming agent is too much, excessive gas generation will cause rapid growth of the pressure in the bubble, excessive growth of cell size, and rapid release of gas, which will damage the cell structure, uneven distribution of cell size, and even the formation of open cell structure, which will produce large bubbles and cavities locally. During the production of foamed plastic products, exothermic PVC foaming regulators shall not be used alone, but shall be used in combination with endothermic foaming agents or compound chemical foaming agents with heat and exothermic balance. Inorganic foaming agent - sodium bicarbonate (NaHCO3) belongs to endothermic foaming agent. Although the foaming rate is low, the foaming time is long. The mixed use of PVC foaming regulators can play a complementary and balanced role. The exothermic PVC foaming agent improves the gas generation capacity of the endothermic foaming agent, while the endothermic PVC foaming regulator cools the former, stabilizes its decomposition and releases gas evenly, inhibits the internal overheating degradation of thick plates, reduces the precipitation of residues, and has a whitening effect.

On the premise of not affecting the foaming rate, heat absorbing PVC foaming regulator can be appropriately added to replace part of exothermic foaming agent to inhibit the foam breaking caused by adding exothermic foaming agent. 1232 or BLA-616 foaming agent belongs to exothermic and endothermic balanced foaming agent. The decomposition has no induction period, and the decomposition rate is fast. The maximum gas generation volume can be reached in about 10 minutes. The gas release is slow without sudden, and the maximum gas generation volume can reach 156 mL. Its decomposition temperature is within the range of PVC processing temperature. It can be used in the dynamic molding process of products with thick size and complex shape to eliminate foam breaking and ensure the stability of foaming performance.